The invention relates to a method and apparatus for processing video pictures, especially for false contour effect compensation.
More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP) or display devices with digital micro mirror arrays (DMD).
Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat color panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology. On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artefacts which could damage the picture quality. Most of these artefacts are different from the known artefacts occurring on classical CRT color picture tubes. Already due to this different appearance of the artefacts makes them more visible to the viewer since the viewer is used to see the well-known old TV artefacts.
The invention deals with a specific new artefact, which is called xe2x80x9cdynamic false contour effectxe2x80x9d since it corresponds to disturbances of gray levels and colors in the form of an apparition of colored edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation like when the skin of a person is being displayed (e.g. displaying of a face or an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye.
Two approaches have been discussed to compensate for the false contour effect. As the false contour effect is directly related to the sub-field organization of the used plasma technology one approach is to make an optimization of the sub-field organization of the plasma display panels. The sub-field organization will be explained in greater detail below but for the moment it should be noted that it is a kind of decomposition of the 8-bit gray level in 8 or more lighting sub-periods. An optimization of such a picture encoding will have, indeed, a positive effect on the false contour effect. Nevertheless, such a solution can only slightly reduce the false contour effect amplitude but in any cases the effect will still occur and will be perceivable. Furthermore, sub-field organization is not a simple matter of design choice. The more sub-fields are allowed the more complicated will the plasma display panel be. So, optimization of the sub-field organization is only possible in a narrow range and will not eliminate this effect alone.
The second approach for the solution of above-mentioned problem is known under the expression xe2x80x9cpulse equalization techniquexe2x80x9d. This technique is described e.g. in Euro Display 1996, xe2x80x9cAn Equalising Pulse Technique for Improving the Gray Scale Capability of Plasma Displaysxe2x80x9d, K. Toda et al., pages 39 to 42. This technique is a more complex one. It utilizes equalizing pulses which are added or separated from the TV signal when disturbances of gray scales are foreseen. In addition for better compensation quality, since the fact that the false contour effect is motion relevant, different pulses for each possible speed are needed. That leads to the need of a big memory storing a number of big look-up tables (LUT) for each speed and there is a need of a motion estimator. A problem with these equalizing pulses is that they are used to increase or decrease the amplitude of the video signal in area where false contour effect is likely to occur. Thus the correction value is added to the pixel value (RGB data for Plasma Displays) before the corresponding sub-field code word is calculated. Therefore, its not taken into account at which position within the frame period a sub-field is inserted or omitted.
Therefore, it is an object of the present invention to disclose a method and an apparatus which is based on the known solutions using equalizing pulses but which allows for a more efficient false contour effect compensation. This object is achieved by the measures claimed in claims 1 and 4.
The general idea of the invention is that the correction of pixel values is made not on amplitude values only without consideration of the position of the sub-fields which are inserted or omitted but on sub-field level. When the motion in the picture is known for the pixels then the sub-fields for correction are positioned at the best possible location in the frame period for false contour effect compensation.
A correction performed on subfield level allows directly to insert or to remove subfields on the position (time position within the frame) where too much or not enough light impulses are available. This way it""s possible to compensate directly the failures where they occur.
Advantageously, additional embodiments of the inventive method are disclosed in the respective dependent claims.
One example for an apparatus according to the invention is disclosed in claim 3. With a motion estimator the apparatus calculates motion vectors for blocks of pixels of the video frames. It also comprises means for determining critical pixel value transitions which are moving. For given motion vectors and critical pixel value transitions look-up tables are provided in which the corrected digital code words are stored which are to be used for a good false contour effect compensation.